5.2. Effects of Global Change on the Terrestrial Biosphere

Terrestrial ecosystems consist of plants, animals, and soil biota and their
environment. The distribution of biota within and across ecosystems is constrained
by the physical and chemical conditions of the atmosphere, the availability
of nutrients and/or pollutants, and disturbances from natural origin (fire,
wind-throw, etc.) or human land use. Global change affects all of these factors,
but through widely differing pathways and at different scales. Global climate
change, for example, affects local weather and climate in ways that are strongly
dependent on location. Increased atmospheric CO2 concentration, on
the other hand, is geographically more uniform.

At the global scale, the sum of all ecosystems (including the marine biosphere)
exerts a significant role on the balance of carbon and water in the atmosphere.
Feedbacks exist between climate-driven changes in biospheric functioning, such
as enhanced or reduced primary productivity, and the amount of greenhouse gas
(GHG)-related radiative forcing. In addition, changes in land surface characteristics
affect the atmosphere by altering the radiation balance at the reflective surface,
creating further feedbacks. These biospheric feedbacks are discussed in TAR
WGIChapter 3.

In trying to understand the effects of global change on the biosphere as a
whole, scientists often focus on higher level entities such as ecosystems or
biomes (the collection of ecosystems within a particular climatic zone with
similar structure but differing speciese.g., the temperate forest biome).
However, all factors of change act on individual organisms that are part of
a complex web of interactions within and between species and their ecosystem
and within landscapes that contain mosaics of different ecosystems. It is not
feasible to model the impacts of global change at global, or even regional or
landscape, scales at an individual-by-individual level. Thus, most models of
global change have dealt with impacts at the ecosystem or biome level. This
is in contrast with limitations to observational and experimental studies in
which only a selection of individuals and species of a few ecosystems can be
included.